Add multiplexer code.
Add Multiplexer class to encapuslate all the multiplexer operations.
Update comments in the Multiplexer class.
Added conditional so that the class is not defined more than once.
Create a Debugger class to replace the code previously in diagnostics.ino
This version of the code does not yet compile because it calls functions
defined out of scope. This will be resolved once Encoder and MotorController
classes are created.